1. Field of the Invention
The present invention is directed to a method for controlling an expiratory valve in a ventilator and to a computer software product operating in accordance with the invention.
2. Description of the Prior Art
In normal circumstances during respiratory care, the patient must be allowed to exhale in as normally as possible, sometimes against an elevated end expiratory pressure (PEEP). The tubing (the tracheal tube in particular) and devices (e.g. dehumidifier and bacterial filter but especially the ventilator""s expiratory valve) located in the path of flow of expired gas then pose resistance to expiration. The patient is forced to overcome this unnatural resistance, which may be arduous for the patient.
One way to reduce this resistance is to open the expiratory valve to the maximum for a specific period. East German Patent 293 268 describes one such regulatory procedure for a ventilator. In this known procedure, the expiratory valve is an ON/OFF valve with only two positions, completely open or completely closed.
This known regulation of the expiratory valve is accomplished by opening the expiratory valve at the start of expiration. It is then kept open a certain amount of time and then closed. The pressure (end pressure) then present at the valve (on the patient side) corresponds to the pressure in the patient""s lungs. The time the valve should be kept open is determined for subsequent breathing cycles from the difference between the actual end pressure obtained (actual value) and a pre-set pressure for PEEP (reference value). If the actual value is greater than the reference value, the time the valve is kept open is increased. If the actual value is less than the reference value, the time the valve is kept open is reduced. Adjustment toward the reference value is achieved in this manner.
One disadvantage of this known regulatory system is that the patient runs the risk of being subjected to an end pressure lower than PEEP during the initial phase of treatment (when maintaining PEEP is particularly important in preventing the collapse of alveoli in the lungs).
Another disadvantage of this known regulatory system is that the patient is subjected to a varying end pressure, at least during the adjustment phase, since an end pressure higher than the desired PEEP also could be obtained.
A further disadvantage of this known regulatory system is that the patient""s lungs, plus the tubing, does not constitute a static system. Any change in the patient""s position could change the parameters of the gas mechanics of the lungs/tubing system, and the regulatory system would not be able to compensate for this. In a worst case scenario, this could lead to an end pressure far lower (or higher) than the reference value.
Yet another disadvantage is that basic flows cannot be employed, since the known valve is an ON/OFF valve. Basic flows have the advantage of making flow triggering possible for the patient.
Many of these disadvantages can be resolved with a method described in European Application 965 357 (published after the priority date of the present application). This method divides expiration into two phases. In the first phase, the valve is opened enough to reduce flow resistance considerably. In the second phase, the valve is regulated toward the correct end pressure. The duration of the first phase is determined from parameters obtained during the second phase or at the transition between the two phases.
One phenomenon that can, in practice, make it hard to keep an expiratory valve completely open during the first phase (in order to minimize resistance to flow) is that the duration of the second phase becomes too short to allow establishment of the correct end pressure (PEEP). If, in addition, the expiratory valve is non-linear, this could have an adverse impact on regulation in the second phase.
An object of the present invention is to provide a method that avoids the aforementioned problems for controlling an expiratory valve in a ventilator during expiration.
Another object of the invention is to provide a computer software product which e.g. can be used for upgrading existing ventilators and enabling them to perform the functions of in the inventive-method.
The above objects are achieved in accordance with the principles of the present invention in a method, and a computer software product downloadable into a control unit for programming the control unit to operate in accordance with the method, wherein the expiratory valve is maintained fully open in a first interval within an expiratory phase and, in this first interval, flow through and/or pressure at the expiratory valve are measured. The control unit determines if-and when this measured value satisfies a predetermined condition, and a second interval is begun within the expiratory phase if and when the measured value satisfies the predetermined condition. In the second interval, the expiratory valve is regulated to produce a predetermined end pressure in the expiratory section of the ventilator, with which the expiratory valve communicates.
When an expiratory valve is controllable, the valve can be kept fully open for a first interval and then be regulated towards the reference value (PEEP) during a second interval when expiration has largely subsided. Flow through the expiratory valve is determined and utilized for establishing when the; first interval should terminate and when the second interval should begin. In the alternative, the pressure at the expiratory valve is utilized in a similar manner.
It is also possible to utilize a combination of flow and pressure in the expiratory control.
Pressure in the inspiratory part of the ventilator also can be utilized together with flow to determine when to shift from the first interval to the second interval.
Determination of the transition from the first interval to the second interval is appropriately made from threshold values with which the measured flow, pressure or a combination of flow and pressure, is compared. The second interval begins when the threshold value is exceeded. A combination of flow and pressure could e.g. be a calculated estimate of lung pressure, based on measurement values and a model for the system. A threshold value for the estimated lung pressure is then used for determining the transition from the first interval to the second interval.